All but one of the major sewage treatment plants in the Sacramento Delta uses advanced treatment.

The opposite holds true around San Francisco Bay, where advanced treatment methods are the exception, mostly because the discharges are less toxic in saltier waters.

In the Bay Area, the only major sewage treatment plants using tertiary treatment are in the South Bay, which lacks the tides and higher flows of other areas of the bay.

“We haven’t seen a need to require a higher level of treatment (in the bay),” said Lila Tang, program manager for point source wastewater for the San Francisco Bay Regional Water Quality Control Board.

In saltwater, the equilibrium of ammonium and ammonia shifts toward ammonium, which is less toxic to fish and other animals, Tang said.

The bay is not a source of drinking water, either.

And it is cloudier, which makes it more difficult for sunlight to penetrate into the water and increase production of algae, a concern of regulators dealing with nutrient discharges.

Now, however, regulators are faced with the opposite problem: Instead of worrying about nutrients increasing algae production, regulators in the Delta are worried about a particular nutrient — ammonium — suppressing a particular kind of algae — diatoms — that are an important part of the food web.

Tang said regulators in the Bay Area were not rushing to consider higher treatment levels, but added, “It’s something we want to look at more carefully.”

All sewage treatment plants are required to use “secondary” treatment that relies on the use of biological agents to break down pollutants. Advanced, or “tertiary” treatment incorporates filters and other tools to further clean up wastewater.